It is known in the art to determine the purity of semiconductor materials by examining the lifetime of minority carriers therein, in a non-destructive, contact free fashion. A typical prior art system for such measurements is disclosed in U.S. Pat. No. 4,704,576 to Tributsch, et al. (Nov. 3, 1987), and is generally depicted in FIG. 1. In such systems 2, the semiconductor specimen 4 to be tested in placed in a microwave field within or adjacent an open-ended waveguide system 6, between a microwave radiation source 8 (and related circulator 10) and a laser light source 12. Typically the microwave source 8 oscillates at about 10 GHz, with the microwave energy being carried by the waveguide system 6 one-half wavelength (1.5 cm) from the effective antenna 14 to and then from the specimen.
Waveguide system 6 and effective antenna 14 behave like an unterminated stripline antenna system that radiates microwave energy over a large area, without being able to concentrate energy on a given specimen area.
Usually the microwave source frequency is fixed, and the prior art system is tuned by mechanically moving a metal reflector (not shown) to change the microwave phase relationship until the reflected microwave signal (from the detector) is maximized. As shown in FIG. 1, many prior art systems subject one side 16 of the specimen to pulses of energy from the laser source 12, and subject the other specimen side 18 to microwave energy from the microwave source 8. This "two-sided" configuration has several drawbacks. In small resistivity specimens (e.g., &lt;1 .OMEGA.cm), microwave penetration into the specimen is shallow, and thus the recombination phenomena within the specimen induced by the laser excitation at surface 16 is not adequately characterized by microwave energy at the specimen's other side 18. Even in prior art systems that are "one-sided" the inherent system microwave insensitivity hampers meaningful measurements for small resistivity specimens.
Another drawback of "two-sided" configurations as shown in FIG. 1 is that they prevent placing a non-oxidized specimen in an electrolytic bath during measurements. Such a bath serves to passivate non-oxidized specimen surfaces, substantially eliminating surface recombination effects that would otherwise dominate measurements.
The pulsed laser 12 bombards specimen side 16 with pulses of optical energy containing photons that create excess charge carriers within the specimen 4. Sufficient laser energy is present when the wavelength of the laser excitation exceeds the specimen bandgap. In a relatively high resistivity specimen, these carriers can affect the microwave energy reflected by the free electrons and holes in the crystal structure of the specimen. The reflected microwave energy is coupled via circulator 10 to a detector 20. The output from detector 20 allows measurement of the decay time of the optically generated excess carriers. This in turn enables a determination of the recombination time constant of such carriers within the specimen 4. By relatively incrementally repositioning the laser source 12 and specimen 4 in between measurements, the reflecting microwave energy may be used to map defects within the specimen.
More specifically, laser pulse photons that exceed the specimen's bandgap energy are absorbed into the specimen where they generate excess charge carriers, e.g., pairs of mobile electrons and holes that have excess concentrations .DELTA.n and .DELTA.p. These excess carriers increase the specimen conductivity by .DELTA..sigma.: EQU .DELTA..sigma.=q(.mu..sub.n .DELTA.n+.mu..sub.p .DELTA.p)
where q is the electron charge, and .mu..sub.n, .mu..sub.p are respectively the mobility of electrons and holes in the specimen. The excess carrier concentrations .DELTA.n and .DELTA.p decay over time as the carriers become trapped in defects or recombine along defects within the specimen. The excess carriers' time dependent concentration changes the microwave energy reflected from the free electrons and holes within the specimen, which changes are measured with detector 20 (and associated signal processing circuitry). Thus, a measurement of excess conductivity .DELTA..sigma. is indicative of the defects and impurities within the specimen's crystal structure that affect the excess charge carriers.
In the simplest case, recombination is exponential with a reciprocal delay time (1/.tau.) proportional to the concentration of recombination centers, or impurities. Thus 1/.tau. (e.g., recombination lifetime) is a measure of the specimen quality.
Such measurements are especially suitable for materials such as Si that have indirect forbidden bands where the probability of band-band recombination is small. In such semiconductors, the recombination of electrically active impurities, precipitates, interface states formed around secondary phases, scatter centers (e.g., deviations from ideal periodicity) tend to decrease the excess carrier recombination lifetime.
While such prior art systems permit a non-destructive, contactless examination of minority carrier lifetime to provide information as to crystal defects present in the specimen, there are many shortcomings.
A first deficiency arises because microwave energy is coupled to and from the specimen via an open broadbeam waveguide over a relatively large distance (e.g., one-half wavelength). These distances and the broadbeam nature of the waveguide cause a substantial loss in microwave sensitivity, and such prior art systems are characterized by a poor signal/noise ratio. To compensate for such insensitivity, the laser source must be operated at fairly high energy levels. This restriction precludes using high and low laser energy levels for injection spectroscopic measurements to help determine the chemical nature of contaminants in the specimen. This first deficiency is especially troublesome where relatively small resistivity specimens are to be measured.
A second deficiency arises because the relatively large distances in the prior art systems creates standing waves within the microwave waveguide. This causes the specimen to become a tuning element dielectric that, unfortunately, detunes the system with the slightest specimen vibration. As a result, the system requires "resting" after each repositioning of the specimen relative to the laser, to allow the vibrations to dampen before new meaningful measurements can be taken. This "rest time" slows down the rapidity with which measurements may be taken, and complicates the rapid automatic relative repositioning of the specimen and laser source between measurements.
Finally, locating the specimen between the laser and microwave energy sources, as shown in FIG. 1, precludes placing the specimen in an electrolytic bath during measurement. This deficiency arises because an electrolyte bath would attenuate the depth of microwave penetration into side 18 of the specimen such that areas subject to laser illumination from the opposite side 16 are not reached. For a non-oxidized specimen, it would of course be advantageous to permit such testing because an electrolyte would passivate each specimen surface, thereby preventing surface recombination speed from dominating the system measurements. The result, unattainable in prior art configurations such as FIG. 1, would be measurement data providing truer insight as to the condition within the specimen. Of course a non-oxidized specimen could be annealed, typically at elevated temperatures of about 1100.degree. C., to form an oxide layer, which would prevent surface recombination effects from dominating the measurements. However it is well known that such elevated temperatures can produce change in the specimen characteristics.
Thus, there is a need for a non-destructive, contact-free system to characterize semiconductor material that provides high measurement sensitivity, good signal/noise ratio, and permits injection spectroscopic measurements over a wide dynamic range of laser excitation energy. Such system should include a simple mechanism to achieve microwave frequency tuning to optimize system performance.
Further, there is a need for such a system that is substantially immune to specimen vibration, thereby shortening the time between measurements, allowing measurements to be made more rapidly.
Finally, such a system should allow positioning a non-oxidized specimen in an electrolyte bath to allow surface passivation during measurement, thereby permitting a more accurate characterization of the internal structure of the specimen.
The present invention discloses a method and apparatus providing such a system.